Which ones of those can you logically take away that would give tanks an actual role?

There would need to be mechanics changes to allow HAVs to unleash suppressive fire without slaying.

I'm honestly of the opinion that complexes should be owned by infantry and be designed as vehicle death traps (sharp blind corners, multiple angles of attack, lots of cover etc and outer points should be the plaything of vehicles; open, little cover during traversal between complex and outer points etc.

That kind of inaccuracy is uncalled for lol

My thoughts on Vehicles in Legion

First off, I'd like to say that I think all vehicles in Legion should be Crew Served. There should be no "I drive and shoot" vehicles at all (unless the driver is swapping seats). This is a team game and it should encourage teamplay.

Goodness me no, so the entire use of your investment relies on some mltm0 blueberry you don't know and hope he is competent enough to fire on the target you are trying to present him? Vehicles do not have to be crew served as the compulsory, but it shouldn't give 1 person more than 1 forcestrength either.

Alright, now that that is out of the way....

LAVs: Fast, light troop transport intended for small groups looking for fast insertion (that is much less conspicuous than a DS tearing through the sky). They should be the most maneuverable ground vehicles in the game, fairly squishy with only minor offensive potential (single small turret best used to deter enemies from pursuing). Seats three, one driver, one (Small turret) gunner and one passenger (potential to swap turret for extra seat giving one driver and three passengers).

Like the idea of being able to swap out the turet for a seat

MAVs: Durable squad logistical support vehicles intended for use as a mobile command post. Not as fast as LAVs though able to muscle over nearly any terrain, moderately defensible (no need to flee at the first sign of hostiles, though by no means a main battle station). Possesses built in Biomass and Nanite bays, seats six passengers (plus one driver, one main (Medium turret) gunner and two secondary gunners) with enough cargo space to accommodate their gear.

Nope, a MAV is literally an APC it jist won't have the size or the tank to be a command post. 6 passenger, 1 driver operated light/medium turret will be more than sufficient.

HAVs: Beastly direct action support vehicles intended for use as point defense/mobile artillery. Decent ground speed (3/4 MAV speed at max), terrible turret tracking speed (at least outside of Siege), ridiculous EHP and massive DPH potential. Seats one driver, one main (large turret) gunner and two secondary gunners. Can fit a Siege module which consume fuel though increases turret tracking speed at the cost of mobility (activating a Siege mod renders the HAV immobile until the Siege cycle is over).

Fuel? So now we have fuel? Seige modules typically reduce your speed to 5% of its maximum, reduce tracking speeds, but increasd damage and resistance, however seige modes typically time to deactivate meaning a seiged vehicle is often doomed in a poor engagement

DSs: Fast (max speed 3x that of an LAV), fragile, maneuverable airborne troop transports capable of moving up to two squads of passengers faster than any other mode of transport. No turrets though equipped with chaff to assist in longevity as well as a tractor module for those times when the extraction point is too hot for a landing. Seats a single pilot in addition to twelve passengers.

LZ's will always be hot becausd you have no weapons from which ypur team can defend yourself, dropships are typically tactical troop transports, taking no more than 6, but allowing you to drop them off in presice locations, which is better than HALO jumping from a hercules class transport. 6 passengers, 2 light turrets

ADSs: Gunships that are slightly slower than DSs (3/4 DS speed at max), less maneuverable, fairly durable (~150% DS EHP) and bristling with guns though with no passengers. Seats one pilot, one main (Medium turret) gunner and four secondary gunners.

if anything an ADS should have less EHP and more maneuverability, it's an assault model. Once again there is no need to change it, it's too Assault Forwarded

Just a quick runthrough on how I believe vehicles should be (not exactly, though at least the idea is conveyed). All vehicles should have a defined role that isn't just "I invincibly pwn everything".

Let's see:

1: Pilot skills for and pays for (most of the time) for the HAV.

2: Pilot calls in the HAV.

3: lore explains why the pilot controls both the turret and the driving

4: It'd be boring just to drive the HAV, and not get to shoot.

Since you couldn't use the search function, I gave them to you.

1 medium AND 1 large would be OP.